1. Field of the Invention
The present invention relates generally to interbody spinal fusion implants that are securely placed into the intervertebral space created across the spinal disc between two adjacent vertebral bodies after the removal of damaged spinal disc material and preferably at least some vertebral bone from each of the adjacent vertebral bodies for the purpose of achieving interbody spinal fusion, which fusion occurs preferably at least in part through the spinal fusion implant itself. In particular, the present invention is directed to an improved, interbody spinal fusion implant having opposed arcuate surfaces for penetrably engaging each of the vertebral bodies adjacent a disc space in the human spine and having a trailing end configured to conform to the anatomic contour of the anterior and/or lateral aspects of the vertebral bodies, so as to not protrude beyond the curved contours thereof, and in one preferred embodiment of the present invention the above described implants are structurally adapted to be rotated for proper insertion.
2. Description of the Related Art
Surgical interbody spinal fusion generally refers to the methods for achieving a bridge of bone tissue in continuity between adjacent vertebral bodies and across the disc space to thereby substantially eliminate relative motion between the adjacent vertebral bodies. The term "disc space" refers to the space between adjacent vertebrae normally occupied by a spinal disc.
Human vertebral bodies have a hard outer shell of compact bone (sometimes referred to as the cortex) and a relatively softer, inner mass of cancellous bone. Just below the cortex adjacent the disc is a region of bone referred to herein as the "subchondral zone". The outer shell of compact bone (the boney endplate) adjacent to the spinal disc and the underlying subchondral zone are together herein referred to as the boney "end plate region" and, for the purposes of this application, is hereby so defined to avoid ambiguity. A circumferential ring of dense bone extends around the perimeter of the endplate region and is the mature boney successor of the "apophyseal growth ring". This circumferential ring comprises of very dense bone and for the purposes of this application will be referred to as the "apophyseal rim". The spinal disc that normally resides between the adjacent vertebral bodies maintains the spacing between those vertebral bodies and, in a healthy spine, allows for the normal relative motion between the vertebral bodies.
Reference is made throughout this Background section to the attached drawings in order to facilitate an understanding of the related art and problems associated therewith. In FIG. 1, a cross-sectional top plan view of a vertebral body V in the lumbar spine is shown to illustrate the dense bone of the apophyseal rim AR present at the perimeter of the vertebral body V about the endplate region and an inner mass of cancellous bone CB. The structure of the vertebral body has been compared to a core of wet balsa wood encased in a laminate of white oak. From the top plan view in FIG. 1, it can be seen that the best structural bone is peripherally disposed.
FIG. 2 is a top plan view of a fourth level lumbar vertebral body V shown in relationship anteriorly with the aorta and vena cava (collectively referred to as the "great vessels" GV).
FIG. 3 is a top plan view of a fifth lumbar level vertebral body V shown in relationship anteriorly with the iliac arteries and veins referred to by the designation "IA-V". The location of these fragile blood vessels along the anterior aspects of the lumbar vertebrae makes it imperative that no hardware protrude dangerously therefrom where the vessels could be endangered.
Implants for use in human spinal surgery can be made of a variety of materials such as surgical quality metals, ceramics, plastics and plastic composites, cortical bone and other materials suitable for the intended purpose, and further may be absorbable and or bioactive as in being osteogenic. Fusion implants preferably have a structure designed to promote fusion of the adjacent vertebrae by allowing bone to grow through the implant from vertebral body to adjacent vertebral body to thereby fuse the adjacent vertebrae. This type of implant is intended to remain indefinitely within the patient's spine or if made of bone or other resorbable material to eventually be replaced with the patient's bone.
Michelson, Ray, Bagby, Kuslich, and others have taught the use of hollow, threaded perforated cylinders to be placed across a disc space between two adjacent vertebrae in the human spine to encourage interbody spinal fusion by the growth of bone from one vertebra adjacent a disc to the other vertebra adjacent that disc through such implants. Michelson, Zdeblick and others have also taught the use of similar devices that either have truncations of their sides such that they are not complete cylinders, and/or are tapered along their longitudinal axis much like a cylinder which has been split longitudinally and then wedged apart. All of these implants have in common opposed arcuate surfaces for penetrably engaging into each of the vertebral bodies adjacent a disc space to be fused. Such implants now in common use throughout the spine, may be used individually or inserted across the disc space in side-by-side pairs, and may be insertable from a variety of directions.
It is commonly held by surgeons skilled in the art of spinal fusion that the ability to achieve spinal fusion is inter alia directly related to the vascular surface area of contact over which the fusion can occur, the quality and the quantity of the fusion mass (e.g. bone graft), and the stability of the construct. However, the overall size of interbody spinal fusion implants is limited by the shape of the implants relative to the natural anatomy of the human spine. For example, such implants cannot dangerously protrude from the spine where they might cause injury to one or more of the proximate vital structures including the large blood vessels.
With reference to FIG. 4, a top plan view of the endplate region of a vertebral body V is shown to illustrate the area H available to safely receive an implant(s) inserted from the anterior aspect (front) of the spine, with the blood vessels retracted.
As can be seen in FIG. 5, a top plan view of the endplate region of a vertebral body V with the outlines of two differentially sized prior art implants A and B installed, one on each side of the midline of the vertebral body V, are shown. The implantation of such prior art implants A and B is limited by their configuration and the vascular structures present adjacent anteriorly to the implantation space. For example, the great vessels GV present at the L.sub.4 level and above are shown in solid line in FIG. 5, and for the L.sub.5 and S.sub.1 levels, the iliac artery and vein IA-V are shown in dotted line. As shown in FIG. 5, prior art implant A represents an attempt by the surgeon to optimize the length of the implant which is inhibited by a limiting corner LC. Implant A, the longest prior art implant that can be inserted without interfering with the great vessels GV adjacent the vertebral body V, leaves cross-hatched area X of a cross section the vertebral body at the endplate region wasted which would be a very useful surface for contact for fusion and for support of the implant by the vertebral body. Similarly, implant B is an attempt by the surgeon to optimize the width of an implant which is also inhibited by a limiting corner LC'. Implant B, the widest prior art implant that can be inserted without interfering with the great vessels GV adjacent the vertebral body V, leaves cross-hatched area Y of the cross section of the vertebral body adjacent the endplate region wasted which could otherwise be a very useful surface area for contact for fusion and for support of the implant by the vertebral body. The presence of limiting corners LC and LC' on any such implants precludes the surgeon from safely utilizing an implant having both the optimal width and length, that is the length of implant A and the width of implant B combined, as such an implant would markedly protrude from the spine and endanger the large blood vessels.
FIG. 5 illustrates the maximum dimensions for the above discussed prior art implants A and B to be safely contained within the spine so that a corner LC or LC' of the trailing end (side wall to trailing end junction) or the most rearward extension of that sidewall does not protrude outward beyond the rounded contour of the anterior (front) or the anterolateral (front to side) aspect of the vertebral bodies. Prior art implant A maximizes length, but sacrifices width and for the most part fails to sit over the best supportive bone peripherally of the apophyseal rim as previously shown in FIG. 1. Prior implant B maximizes width, but sacrifices length and again fails to sit over the best structural bone located peripherally in the apophyseal rim of the vertebral body, comprising of the cortex and dense subchondral bone. Both prior art implants A and B fail to fill the area available with a loss of both vital surface area over which fusion could occur and a loss of the area available to bear the considerable loads present across the spine.
Similarly, FIG. 6A shows the best prior art cross-sectional area fill for a pair of inserted threaded implants G as per the current prior art. Note the area Y anterior to the implants G, including the excellent structural bone of the apophyseal rim AR, is left unused, and thus implants G fail to find the best vertebral support. Since the wasted area Y anterior to the implants G is three dimensional, it also wastes a volume that optimally could be utilized to hold a greater quantity of osteogenic material. Finally, the implants of the prior art fail to achieve the optimal stability that could be obtained by utilizing the greater available surface area of contact and improved length that an implant with the maximum width and length would have, and thereby the best lever arms to resist rocking and tilting, and increased contact area to carry further surface protrusions for providing stability by engaging the vertebrae, such as with the example shown of a helical thread.
FIG. 11 shows the best fill obtained when a prior art implant C is inserted, from a lateral approach to the spine (from a position anterior to the transverse processes of the vertebrae) referred to herein as the "translateral approach" or "translaterally" across the transverse width W of vertebral body V. Some examples of implants inserted from the translateral approach are the implants disclosed in U.S. Pat. No. 5,860,973 to Michelson and preferably inserted with the method disclosed in U.S. Pat. No. 5,772,661 to Michelson. Implant C does not entirely occupy the cross-sectional area of the end plate region and leaves cross-hatched area Z of the vertebral body V unoccupied by the implant which area would be useful for contact for fusion and for support of the implant. The configuration of the trailing corner LC" of the prior art implant C prevents implant C from being sized larger and prevents the full utilization of the surface area of contact of the vertebral body cross-sectional area resulting in a sub-optimal fill of the disc space with the implant, and little of the implant sitting on the apophyseal rim.
The configuration of prior art implants prevents the utilization of the apophyseal rim bone, located at the perimeter of the vertebral body to support the implants at their trailing ends. The utilization of this dense bone would be ideal.
Therefore, there is a need for an interbody spinal fusion implant having opposed arcuate portions for penetrably engaging adjacent vertebral bodies, including implants requiring rotation for proper insertion into an intervertebral space formed across the disc space between two adjacent vertebrae, that is capable of fitting within the external perimeter of the vertebral bodies between which the implant is to be inserted to maximize the surface area of contact of the implant and vertebral bone without the danger of interfering with the great vessels adjacent to the vertebrae into which the implant is to be implanted. There exists a further need for an implant that is adapted to utilize the dense cortical bone in the perimeter of the vertebral bodies in supporting such an implant installed in a disc space.